The goals of this project are to test specific hypotheses about the pathophysiology of obsessive compulsive disorder (OCD) using functional MRI (fMRI) and cognitive neuroscience methods. The basal ganglia have been implicated in the pathophysiology of OCD and also in the normal mediating anatomy of implicit (i.e., nonconscious) learning. We have developed an fMRI paradigm to measure activity within components of the basal ganglia during the performance of an implicit learning task. In preliminary functional imaging studies, we have observed an abnormal activation profile in medication-free subjects with current OCD. Specifically, in comparison to normal control subjects, patients with OCD exhibit a failure to activate right striatum, and an absence of thalamic deactivation, as well as aberrent medial temporal activation (i.e., not found in normal subjects). These results are consistent with evolving neurobiological models of OCD and basal ganglia function. The current proposal aims to: 1) use this fMRI paradigm to replicate and elaborate upon the above findings in a larger cohort of medication-free patients with active OCD and matched normal comparison subjects; 2) determine the specificity of the findings in OCD by studying psychiatric comparison subjects; and 3) further investigate the neuropsychological consequences of the abnormal activation profile observed in OCD. It is anticipated that this approach will yield new information regarding the pathophysiology of OCD and related disorders, while providing new insights about the normal function of the basal ganglia and the mediating anatomy of normal learning.